1. Field of the Invention
The present invention relates to a developing device used in an image forming apparatus such as copying machine, printer or facsimile that utilizes electronic photography technique and, more particularly, to a developing device of non-contact developing system wherein a toner included in a developing agent held onto a magnetic roller is caused to deposit on the surface of a developing roller and then move onto an electrostatic latent image, thereby developing the electrostatic latent image.
2. Description of Related Art
There have been various methods employed by image forming apparatuses that utilize electronic photography technique. Single-drum color superposition system where images of different colors are formed successively on a single photosensitive body has been regarded as a promising technique that makes high-quality color imaging possible, since it allows full-color image formation with less color registration error by precisely depositing the toners of different colors one after another on the photosensitive body.
In recent years, meanwhile, much attention has been focused on tandem-drum system where images are formed with toners of different colors on a plurality of corresponding photosensitive bodies, and the images formed on the respective photosensitive bodies are superposed one after another on a transfer member in synchronization with feeding of the transfer member, thereby to form a full-color image. Although this system has an advantage of high-speed operation, there is such a disadvantage that one set of imaging means such as photosensitive bodies and a developing device must be provided for each color and arranged in tandem, thus resulting in larger size of the apparatus.
To counter this problem, such a compact tandem engine image forming apparatus is proposed that is provided with imaging means made smaller in size by decreasing the space between the photosensitive bodies. In the compact tandem engine image forming apparatus, it is advantageous to dispose the developing device in vertical construction in order to decrease the horizontal size of the imaging means. In other words, it is preferable that the members such as the developing roller, a magnetic roller, stirring-transfer means, etc. that constitute the developing device are disposed right above or obliquely above the photosensitive body, rather than in the same plane as the photosensitive body as in the case of the prior art.
In the case of a developing system that employs a two-component developing agent, however, decreasing the apparatus in size results in such problems as the deposition of carrier onto the photosensitive body and scattering of the toner. When the developing device is made smaller in vertical construction, in particular, circulation, of the developing agent, namely supply of the developing agent from a position near the stirring-transfer means to the developing roller, becomes so complicated that the problems of deposition of the carrier onto the photosensitive body and scattering of the toner become more conspicuous, thus imposing a limitation to the size reduction of the device.
Means for solving the problems of deposition of the carrier onto the photosensitive body are described in Patent Documents 1 through 3 described below that propose a developing device and an image forming apparatus provided with the developing device (hybrid developing system). The developing device comprises stirring-transfer means, a magnetic roller and a developing roller that are disposed in a housing, which contains a two-component developing agent. The stirring-transfer means transfers the developing agent while stirring it. The magnetic roller transfers the developing agent while holding it on the circumferential surface thereof. The developing roller is disposed to oppose the magnetic roller at a predetermined space therefrom and attracts the toner included in the developing agent held by the magnetic roller to deposit electrostatically thereon. With this system, deposition of the carrier onto the photosensitive body is suppressed and the problem of depletion of the carrier becomes less likely to occur, since the carrier does not fly toward the photosensitive body and returns to the magnetic roller even when a small amount of carrier moves from the magnetic roller to the developing roller and deposits thereon.
However, in the hybrid developing system described above, residual toner that has not been used in development and remains on the developing roller is scraped off by a magnetic brush provided on the magnetic roller, while some of the residual toner may not be completely cleaned and remain on the developing roller. When the residual toner is not completely cleaned, part of the previous picture may remain as a ghost in the next development process in the so-called hysteresis phenomenon (refer to FIG. 5). If the residual toner is not completely cleaned after development, there also occurs a problem that the toner sticks onto the developing roller and does not fly to the photosensitive body.    [Patent Document 1] Japanese Unexamined Patent Publication (Kokai) No. 6-67546 (FIGS. 1 and 2)    [Patent Document 2] Japanese Unexamined Patent Publication No. 2003-295613    [Patent Document 3] U.S. Pat. No. 3,929,098